This invention relates to semiconductor devices, particularly to field-effect semiconductor devices as typified by the high electron mobility transistor (HEMT), the two-dimensional electron gas (2DEG) diode (diode that utilizes a layer of 2DEG as a current path or channel), and the metal-semiconductor field-effect transistor (MESFET). More particularly, the invention pertains to such field-effect semiconductor devices that are normally off, instead of normally on as in the case of most of such devices known heretofore. The invention also deals with a method of fabricating such normally-off field-effect semiconductor devices.
The HEMT, a type of FET, has been known and used extensively which comprises an electron transit layer overlying a substrate via a buffer, and an electron supply or barrier layer on the electron transit layer. The electron transit layer is of undoped semiconducting nitride such as gallium nitride (GaN) grown on a silicon or sapphire substrate. The electron supply layer is of either n-doped or undoped semiconductor nitride such as aluminum gallium nitride (AlGaN) deposited on the electron transit layer. A source, a drain and a gate (Schottky) electrode are all disposed on the electron supply layer.
Made from the dissimilar semiconducting materials as above, the electron transit layer and electron supply layer differ in both bandgap and lattice constant. The electron supply layer of AlGaN or the like is greater in bandgap, and less in lattice constant, than the electron transit layer of GaN or the like. By being placed on the electron transit layer having a greater lattice constant, the electron supply layer gives rise to an expansive strain or tensile stress and hence to piezoelectric depolarization. The electron supply layer is additionally subject to spontaneous depolarization.
Consequently, due to both piezoelectric and spontaneous depolarizations, what is known as 2DEG, a gas of electrons free to move in two dimensions only, appears along the heterojunction between the electron supply layer and the electron transit layer. The 2DEG provides a channel of very low resistance, or of high electron mobility, for source-drain current flow. This current flow is controllable by a bias voltage applied to the gate electrode.
As heretofore constructed, the HEMTs of the general construction above were mostly normally on; that is, there was a source-drain flow of electrons while no voltage was applied to the gate. The normally-on HEMT had to be turned off using a negative power supply for causing the gate electrode to gain a negative potential. Use of such a negative power supply made the associated circuitry unnecessary complex and expensive. The conventional normally-on HEMTs were rather inconvenient of use.
Attempts have been made to develop normally-off heterojunction FETs. Among the suggestions heretofore made toward this end are:
1. Accommodating the gate in a recess that is formed in the electron supply layer to make this layer thinner at the recessed part.
2. Interposing a p-type nitride semiconductor layer between the gate and the electron supply layer (Japanese Unexamined Patent Publication No. 2004-273486).
3. Accommodating the gate in the recess (ditto) in the electron supply layer via an insulating film of strontium titanate or the like (Japanese Unexamined Patent Publication No. 2006-222414).
4. Partly removing the electron supply layer to expose part of the underlying electron transit layer and placing the gate on the exposed part of the electron transit layer via an insulating film (WO 2003/071607).
The first cited known scheme is designed to weaken the electric field due to the piezoelectric and spontaneous depolarizations at and adjacent the part of the electron supply layer that has become thin by recessing. The weaker electric field is cancelled out by the built-in potential of the device, that is, the potential difference between the gate and the electron supply layer when no bias voltage is being applied to the gate. The result is the disappearance of the 2DEG from the neighborhood of the gate. The remaining 2DEG is unable to convey source-drain current while no voltage is being applied to the gate. The device is therefore normally off.
The HEMT built on this first known scheme had several shortcomings. First, it had a threshold voltage as low as one volt or even less and so was easy to be triggered into action by noise. What was worse, this threshold changed substantively with manufacturing errors in the depth of the recess in the electron supply layer. Another weakness was that there was a relatively large current leakage upon application of a positive voltage to the Schottky gate. Creating the recess in the electron supply layer brought about the additional weakness that the resulting thin part of the electron supply layer was not necessarily satisfactory in its capability of supplying a sufficient amount of electrons to the electron transit layer when a voltage was applied to the gate in order to turn the device on. The fact that the 2DEG channel was partly insufficient in electron density made the turn-on resistance of the device inordinately high.
The provision of the p-type semiconductor layer under the gate according to the second known suggestion above serves to raise the potential of the underlying part of the electron transit layer. The resulting depletion of electrons from under the gate creates a hiatus in the 2DEG layer, making the device normally off.
An objection to this second known suggestion is the difficulty of creating the p-type nitride semiconductor layer of the sufficiently high hole density required. In cases where this requirement was not met, then it became necessary either to make the electron supply layer thinner or, if the electron supply layer was made from AlGaN or aluminum indium gallium nitride (AlInGaN), to lower its aluminum content. The result in either case was a drop in the electron density of the 2DEG layer, which in turn made higher the source-drain turn-on resistance.
The third known solution also seeks to gain normally-off performance by making the electron supply layer thinner under the gate by creating a recess in that layer. By receiving the gate in this recess via the insulating film, the HEMT is saved from a rise in current leakage and made higher in transconductance (symbol gm).
Having a recess in the electron supply layer, this prior art normally-off HEMT possesses the same shortcomings as the first described one. As an additional disadvantage, the insulating film was susceptible to physical defects, particularly when made thinner for a higher transconductance. A defective insulating film was a frequent cause of current leakage, lower antivoltage strength, device destruction, and current collapse. All these results would be avoidable by making the insulating film thicker, but then the device would be inconveniently low in transconductance.
With the gate placed on the exposed part of the electron transit layer via an insulating film as in the fourth known solution above, the 2DEG in the electron transit layer is normally absent from under the gate. A problem with this prior art device arose when the gate was excited to turn it on. Its turn-on resistance was higher than the more conventional normally-on HEMT by reason of the absence of the 2DEG from under the gate.
The difficulties and inconveniences pointed out hereinbefore in conjunction with the known normally-off field-effect semiconductor devices are not limited to HEMTs alone. Similar problems have attended the conventional endeavors to provide other types of normally-off 2DEG diodes and MESFETs and like field-effect semiconductor devices other than HEMTs.